This invention relates to programmable logic devices (PLD) and in particular to folded PLDs that are user programmable.
U.S. Pat. No. 4,872,137 includes disclosure of a folded programmable logic device employing a triangular geometry, using an AND-gate array in which reprogrammable control circuits (RCC) are used to select and control programmable inputs to the AND gates. The RCC design is based on a transmission gate controlled by the user programmable content of a 1-bit shift register. The triangular geometry permits more effective use of semiconductor chip area to implement folded programmable logic devices than use of rectangular arrays employing previously proposed folding techniques. However, while this PLD design is advantageous in terms of versatility of application, operating speed and optimization of AND-gate utilization, it would be desirable to achieve a PLD structured to enable even greater versatility in terms of ability to process more complex Boolean logic inputs. Also, it is desirable to facilitate implementation of user programmable complex logic functions.